Modulators typically modulate baseband data prior to transmission by a radio transmitter in response to receiving the baseband data from a baseband processor. As used herein, modulation of baseband data also refers to encoding of the baseband data if required. Accordingly, a modulator converts the baseband signal into a symbol, based on a mapping of the baseband data into one of a set of symbols as part of the process of modulating the baseband signal. For example, phase shift keyed (PSK) modulators are known to receive symbol data and to convert the symbol data into phase data using M-phase PSK modulation, where M represents the number of possible phases corresponding to the size of the symbol set.
According to one method, a modulator maps three consecutive bits of data from a baseband processor into one of eight symbols in an 8 phase PSK (8PSK) modulation format known as Gray coding. However, phase transitions for sequentially adjacent phase symbols that are 180 degrees, or π radians out of phase, exhibit zero crossings on a phase signal constellation map. Sequentially adjacent phase shift keyed signals that exhibit zero crossings in the phase signal constellation map create higher bandwidth signal components, such as intermodulation signals and other spurious signals, as opposed to sequentially adjacent signals that do not cross zero in the phase constellation map. As a result, phase shift keyed signals that exhibit zero crossings require the use of a very linear power amplifier in order to transmit the large bandwidth modulated signal.
Phase shift keyed signal transitions for sequentially adjacent phase symbols that exhibit zero crossings in the phase signal constellation map also result in a high peak to average and high peak to minimum power ratio compared to signal transitions that do not cross zero in the phase signal constellation map. Modulated signals with a high peak to average and high peak to minimum power ratio may saturate a power amplifier, creating greater interference noise in adjacent channels of a communication system. The interference noise in adjacent channels can reduce communication system capacity.
According to a second method, to minimize saturation of the power amplifier as a result of a modulated signal with a high peak to average and peak to minimum power ratio, the operating region of the power amplifier is reduced, which in turn reduces the power amplifier efficiency and further increases power consumption of the power amplifier. Consequently, modulated signals with high peak to average and peak to minimum power ratios require large bandwidths and additional power to operate, resulting in, for example, reduced battery life in a wireless device.
According to a third method, each symbol may be rotated when mapping the baseband data to a symbol; for example, each symbol may be rotated by a multiple of a predetermined phase rotation amount when using M-phase PSK modulation. This phase rotation reduces the zero crossings on the phase signal constellation map for sequentially adjacent phase modulated symbols, resulting in a modulated signal with reduced bandwidth and a reduction in the peak to average and peak to minimum power ratio of phase modulated signal. However, the phase rotation amount is not programmable in the modulator and, as a result, remains fixed. Consequently, this type of modulator cannot be used to communicate with modulation formats of different phase rotations, as used in different communication systems.
According to a fourth method, each symbol may be rotated by adding a fixed number to the symbol data to form a rotated symbol. The rotated symbol is then converted to an amount of rotation as a phase angle via lookup table. However, the rotation amount is not based on a symbol count to ensure that sequentially adjacent phase symbols do not exhibit zero crossings on a phase signal constellation map. Rather, the rotation amount is derived by adding a fixed number to each symbol received in the symbol data. Additionally, the rotation amount is not programmable in the modulator. Consequently, this type of modulator cannot be used to communicate with modulation formats of different phase rotations, as used in different communication systems.